Low dielectric constant film material, film and semiconductor device using such material

ABSTRACT

A low dielectric film forming material contains siloxane resin and polycarbosilane dissolved in solvent. By using this solution, a low dielectric film is formed which contains siloxane resin and polycarbosilane bonded to the siloxane resin. Material of a low dielectric film is provided which is suitable for inter-level insulating film material. A semiconductor device is also provided which has a low dielectric constant film and high reliability.  
     This application is based on Japanese Patent Applications  2000 - 92138  filed on Mar.  29, 2000,  and  2001 - 2113  filed on Jan.  10, 2001,  the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0001] a) Field of the Invention

[0002] The present invention relates to low dielectric constant filmmaterials, and to low dielectric constant films and semiconductordevices using such materials.

[0003] b) Description of the Related Art

[0004] Higher integration and higher speed of semiconductor integratedcircuits are desired. A signal transmission speed in a semiconductorintegrated circuit is restricted by wiring resistance and parasiticcapacitance between wiring lines. Wiring resistance and parasiticcapacitance are increasing as the wiring width and pitch are becomingnarrow because of higher integration of semiconductor integratedcircuits. Although parasitic capacitance can be reduced by thinningwiring lines, wiring resistance increases and the reduced capacitancedoes not contribute to improving the signal transmission speed. In orderto increase the signal transmission speed, it is effective to make aninter-level insulating film have a low dielectric constant.

[0005] Conventionally, an inter-level insulating film is made ofnon-organic material such as silicon dioxide (SiO₂), silicon nitride(SiN), silicon oxyfluoride (SiOF) and phosphosilicate glass (PSG), ororganic polymer such as polyimide. A relative dielectric constant of asilicon dioxide film formed by chemical vapor deposition (CVD) is about4. A relative dielectric constant of a SiOF film is about 3.3 to 3.5which is lower than that of silicon dioxide. However, since SiOF has ahigh moisture absorption rate, a SiOF film is likely to absorb moistureand raise its relative dielectric constant.

[0006] As low dielectric constant materials, attention has been paid tosiloxane resin having Si—H bonds, porous siloxane resin or the like.

[0007] As siloxane resin is washed with alkaline solution, SiOH having ahigh moisture absorption rate is produced because of hydrolysis.Therefore, siloxane resin washed with alkaline solution raises itsrelative dielectric constant. Apart from this, an organic polymer filmhas as low a glass transition temperature as about 200 to 350° C. and ahigh thermal expansion coefficient. Damages to wiring layers aretherefore large.

SUMMARY OF THE INVENTION

[0008] It is an object of the present invention to provide materials oflow dielectric constant films suitable for inter-level insulatingmaterials.

[0009] It is another object of the invention to provide low dielectricconstant films suitable for inter-level insulating films.

[0010] It is still another object of the invention to providesemiconductor devices having low dielectric constant films and highreliability.

[0011] According to one aspect of the present invention, there isprovided a low dielectric constant film forming material containingsiloxane resin and polycarbosilane dissolved together.

[0012] According another aspect of the invention, there is provided alow dielectric constant film made of siloxane resin and polycarbosilanebonded to the siloxane resin.

[0013] According to another aspect of the invention, there is provided asemiconductor device comprising a semiconductor substrate and adielectric film disposed on the principal surface of the semiconductorsubstrate and made of low dielectric constant material containingsiloxane resin and polycarbosilane bonded to the siloxane resin.

[0014] By adding polycarbosilane to siloxane resin, resistance againstalkaline of a siloxane resin film can be improved.

[0015] According to another aspect of the present invention, there isprovided a semiconductor device comprising: a semiconductor substrate; afirst film formed on a surface of the semiconductor substrate and madeof a first silica-containing porous material; and a second film directlyformed on the first film and made of a second silica-containing porousmaterial, the second silica-containing porous material having an etchingrate different from an etching rate of the first silica-containingporous material under a same etching condition.

[0016] According to another aspect of the invention, there is provided amethod of manufacturing a semiconductor device, comprising the steps of:forming a first film made of a first silica-containing porous materialon a surface of a semiconductor substrate; forming a second film of asecond silica-containing porous material directly on a surface of thefirst film, an etching rate of the second silica-containing porousmaterial being faster than an etching rate of the firstsilica-containing porous material; forming a trench having a depthgreater than a thickness of the second film and a via hole through thefirst film, the via hole being partially overlapped by the trench; andburying a conductive material in the via hole and trench.

[0017] By using silica-containing porous material as the material of thefirst and second films, a dielectric constant can be lowered. Since theetching rates of the first and second films are different, it is easy toselectively etch only one of the films.

[0018] As above, by adding polycarbosilane to siloxane resin, resistanceagainst alkaline of a low dielectric constant film can be improved.Therefore, a dielectric constant of a low dielectric constant film canbe maintained low even after a process using alkaline solution.

[0019] By stacking a film made of silica-containing porous materialhaving a faster etching rate upon a film made of silica-containingporous material having a slower etching rate, etching the upper layercan be stopped with relatively good controllability when the lower filmis exposed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a graph showing a relation between polycarbosilaneaddition amounts and relative dielectric constants of films made of lowdielectric constant film materials according to first to fourthembodiments.

[0021]FIG. 2 is a graph showing a relation between polycarbosilaneaddition amounts and adhesion degrees of films made of low dielectricconstant film materials according to the first to fourth embodiments.

[0022]FIG. 3 is a graph showing a relation between polycarbosilaneaddition amounts and relative dielectric constants of films made of lowdielectric constant film materials according to fifth to eighthembodiments.

[0023]FIG. 4 is a graph showing a relation between polycarbosilaneaddition amounts and adhesion degrees of films made of low dielectricconstant film materials according to the fifth to eighth embodiments.

[0024]FIG. 5 is a cross sectional view of a semiconductor device usinglow dielectric constant films which are made of materials of any of thefirst to eighth embodiments.

[0025]FIG. 6 is a cross sectional view of a semiconductor device usinglow dielectric constant films according to a ninth embodiment of theinvention.

[0026]FIG. 7 is a cross sectional view of a semiconductor device usinglow dielectric constant films according to a tenth embodiment of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] Materials of a low dielectric constant film used in embodimentsof the invention are obtained by dissolving siloxane resin andpolycarbosilane region in solvent.

[0028] Siloxane resin may be those materials expressed by the followinggeneral chemical formula:

[0029] R₁ to R₃ represent hydrogen, oxygen or a monovalent hydrocarbongroup such as a methyl group, an ethyl group and a phenyl group, and Xrepresents hydrogen or Si. The number n₁ of monomer units is 5 to 200.If R₁ to R₃ are hydrogen, the group similar to X is bonded to theseoxygen atoms. If X is Si, a main chain (—Si—O—) extends from this Siatom. The resin expressed by the general chemical formula may be: resinformed by a sol-gel process by using, as source material,tetraalkoxysilane, trialkoxysilane, methyltrialkoxysilane, or the like;resin formed by a sol-gel process by using a mixture of these sourcematerials; resin formed by a sol-gel process by using, as sourcematerial, tetraalkoxysilane and dimethylalkoxysilane; and other resin.

[0030] Siloxane resin may also be those ladder type materials expressed,for example, by the following general chemical formula:

[0031] At least one of R₄ to R₇ represents hydrogen, and the othersrepresent hydrogen, oxygen or a monovalent hydrocarbon group such as amethyl group, an ethyl group and a phenyl group. The number n₂ ofmonomer units is 5 to 100. The resin expressed by the general chemicalformula may be hydrogen silsesquioxane, methyl silsesquioxane,fluorine-containing hydrogen silsesquioxane or the like.

[0032] Polycarbosilane may be those materials expressed by the followinggeneral chemical formula:

[0033] R₈ and R₉ represent hydrogen or a monovalent hydrocarbon groupsuch as a methyl group, an ethyl group and a phenyl group, and Xrepresents hydrogen or Si. The number m of monomer units is 20 to 1,000.

[0034] Usable solvent is not particularly limited if it can dissolvesiloxane resin and polycarbosilane. For example, usable solvent may becyclohexanone, methyl isobutyl ketone, methyl ethyl ketone, methylcellosolve, ethyl cellosolve, octane, decane, propylene glycol,propylene glycol monoethylether, propylene glycol monoethyletheracetate, or the like.

[0035] In order to make a porous low dielectric constant film, organiccompound (desorption agent) which can be desorbed by heat or light maybe added to solvent. Such organic compound may be adamantane compoundsuch as adamantane monophenol, or the like. A porous low dielectricconstant film has a dielectric constant lower than if it is not madeporous. If desorption agent is added too much, the mechanical strengthof the film is lowered. It is therefore preferable to set the additionamount of desorption agent to 70 weight % or less relative to a mixtureof siloxane resin and polycarbosilane.

[0036] The inventors have found that siloxane resin added withpolycarbosilane is given a nature of repelling alkaline solution. A lowdielectric constant film made of the above-described material is easy torepel alkaline solution. Therefore, even if a semiconductor substratewith a low dielectric constant film made of such material is worked inalkaline solution, hydrolysis of the film by alkaline solution can besuppressed and an increase in the dielectric constant can be suppressed.

[0037] Since polycarbosilane has a high compatibility with siloxaneresin, it can be dispersed uniformly in siloxane resin.

[0038] Polycarbosilane has a high moisture resistance. By addingpolycarbosilane to porous siloxane resin which is inferior, particularlyto moisture resistance, the moisture resistance of siloxane resin can beimproved considerably.

[0039] If an average molecular weight of polycarbosilane is too small,most of polycarbosilane is evaporated by heat during the film formation.If the average molecular weight is too large, solubility ofpolycarbosilane relative to solvent lowers so that it is difficult toproduce coating solution. It is therefore preferable to set an averagemolecular weight of polycarbosilane to a value of 1,000 or larger and500,000 or smaller.

[0040] If siloxane resin contains a silanol group having a high moistureabsorption rate, the side chain of polycarbosilane is preferablyhydrogen, because silanol groups reduce by reaction between a silanolgroup and hydrogen.

[0041] If the addition amount of polycarbosilane is too small,sufficient alkaline resistance and moisture resistance cannot beobtained. If the addition amount is too large, an adhesion degree of afilm lowers. It is therefore preferable to set the addition amount ofpolycarbosilane to 10 to 300 weight parts relative to siloxane resin of100 weight parts.

[0042] Low dielectric constant film material described above isspin-coated on the surface of a semiconductor substrate, solvent isevaporated at 120 to 250° C., and heat treatment at 300° C. or higher isperformed for cross-linking. In this manner, a low dielectric constantfilm with polycarbosilane being cross-linked with siloxane resin can beobtained.

[0043] Next, a specific method of producing low dielectric constant filmmaterial according to the first embodiment will be described.

[0044] Tetraethoxysilane of 20.8 g (0.1 mol) and methyltriethoxysilaneof 17.8 g (0.1 mol) are dissolved in methylisobutylketone of 39.6 g.Nitric acid solution of 16.2 g (0.9 mol) at a concentration of 400 ppmis dripped in ten minutes and thereafter an aging process is performedfor two hours. Tetraethoxysilane and methyltriethoxysilane are thereforecopolymerized to produce siloxane resin.

[0045] Next, magnesium nitrate of 5 g is added to remove excessive watercontents. Ethanol produced by the aging process is removed until thereaction solution reduces to 50 ml, by using a rotary evaporator. Methylisobutyl ketone of 20 ml is added to the obtained reaction solution toproduce siloxane resin solution.

[0046] Polycarbosilane of an average molecular weight of 20,000 is addedto the siloxane resin solution by an amount of 10 to 300 weight partsrelative to the siloxane resin (solid composition) of 100 weight parts.With these processes, resin solution to be used for forming a lowdielectric constant film can be produced. Polycarbosilane has the mainchain of (—SiH(CH₃)—CH₂—).

[0047] For the comparison sake, resin solution without polycarbosilaneand resin solution with polycarbosilane at addition amounts of 5 and 350weight parts were formed.

[0048] Next, a method of producing low dielectric constant film materialaccording to the second embodiment will be described.

[0049] In the first embodiment, tetraethoxysilane of 20.8 g andmethyltriethoxysilane of 17.8 g are used as the source material ofsiloxane resin. In the second embodiment, tetraethoxysilane of 20.8 g(0.1 mol) and triethoxysilane of 16.4 g (0.1 mol) are used. Theproduction processes are similar to the first embodiment. Isobutylketone of 37.2 g is used.

[0050] Next, a method of producing low dielectric constant film materialaccording to the third embodiment will be described. Sulfuric acid of 88g (0.9 mol) and fuming sulfuric acid of 33 g (60% SO₄) are introducedinto a reaction chamber having a nitrogen gas inlet pipe and aquantitative liquid pump. Fuming sulfuric acid is introduced in order todehydrate the inside of the reaction chamber. Toluene of 87 g (0.95 mol)is dripped from the quantitative pump at 2 ml/min and thereafter, anaging process is performed for one hour. With this aging process,toluenesulfonic hydrate is produced.

[0051] Solution of trichlorosilane of 41 g (0.3 mol) at a concentrationof 20 weight % dissolved in toluene is dripped from the quantitativepump at 2 ml/min. After dripping, an aging process is performed for twohours. With this aging, ladder type siloxane resin is synthesized. Afterthese processes, fluoric acid solution of 100 ml at a concentration of50 weight % is added to then remove precipitated toluenesulfonic acid.Excessive fluoric acid solution is removed by using a separatory funnel.

[0052] The remaining fluoric acid is neutralized with calcium carbonateof 2 g. After dehydration by magnesium nitrate of 5 g, toluene isremoved by using a rotary evaporator. With these processes, a solidmaterial of hydrogen silsesquioxane resin of 15 g is produced. Thishydrogen silsesquioxane is dissolved in methylisobutyl ketone of 70 g toobtain solution having a solid material concentration of 17.5 weight %.

[0053] Polycarbosilane of an average molecular weight of 20,000 is addedto the obtained solution by an amount of 20 to 300 weight parts relativeto the solid material in the solution of 100 weight parts.

[0054] Next, a method of producing low dielectric constant film materialaccording to the fourth embodiment will be described. In the thirdembodiment, trichlorosilane of 41 g is used as the source material ofhydrogen silsesquioxane. In the fourth embodiment, trichlorosilane 36 g(0.27 mol) and fluorotrichlorosilane of 4.6 g (0.03 mol) are used. Theproduction processes are similar to the third embodiment.

[0055] In the fourth embodiment, resin solution of fluoride-containinghydrogen silsesquioxane of 15 g and polycarbosilane is produced.

[0056] Next, a method of forming a low dielectric constant film by usinglow dielectric constant film material (resin solution) according to anyone of the first to fourth embodiments will be described.

[0057] Resin solution of one of the first to fourth embodiments isspin-coated on a silicon wafer surface at 3,000 rpm in 20 seconds. Afterthe spin-coating, solvent is evaporated at 200° C. In a nitrogenatmosphere which contains oxygen at a concentration of 100 ppm or lower,a heat treatment is performed at 400° C. for 30 minutes. With this heattreatment, siloxane resin and polycarbosilane are cross-linked and a lowdielectric constant film is formed.

[0058]FIG. 1 is a graph showing a relation between relative dielectricconstants of low dielectric constant films and addition amounts ofpolycarbosilane. The abscissa represents the addition amount ofpolycarbosilane relative to siloxane resin of 100 weight parts, in theunit of “weight part”, and the ordinate represents the relativedielectric constant of a low dielectric constant film. In FIG. 1, whitecircle, white square, white triangle and white rhomboid symbols indicatethe relative dielectric constants of low dielectric constant filmsrespectively made of the film forming materials of the first to fourthembodiments. For the reference sake, the relative dielectric constantsof comparison examples without polycarbosilane and with polycarbosilaneat an addition amount of 350 weight parts are shown by using identicalsymbols to those of corresponding embodiments.

[0059] Each of the embodiments shows a relative dielectric constant ofabout 2.5 to 3 which are lower than that of an insulating film made ofsilicon dioxide. Particularly in the first and second embodiments, therelative dielectric constant is made low by adding polycarbosilane.

[0060]FIG. 2 shows a relation between the adhesion degree of a lowdielectric constant film and the addition amount of polycarbosilane. Theadhesion degree was measured with a Sebastian meter by attaching a studpin having a diameter of 2 mm to the film surface with epoxy resin. Theabscissa of FIG. 2 represents the addition amount of polycarbosilane inthe unit of “weight part”, and the ordinate represents a tensilestrength per unit area when peel-off occurs, in the unit of “N/cm²”. Themeaning of each symbol in FIG. 2 is similar to that used in FIG. 1.

[0061] The adhesion degrees of comparison examples with polycarbosilaneadded by 350 weight parts are lower than those of the embodiments withpolycarbosilane added by 300 weight parts or less. It is thereforepreferable to set the addition amount of polycarbosilane to 300 weightparts or less relative to siloxane resin of 100 weight parts.

[0062] Next, an alkaline resistance of a low dielectric constant filmwill be described. The surface state of a low dielectric constant filmwas observed after it is immersed for one minute in tetramethylammoniumhydride solution at a concentration of 2.38%. Cracks formed in the filmswere observed in comparison examples without polycarbosilane andcomparison examples with polycarbosilane of five weight parts. No crackwas observed in the films of the first to fourth embodiments withpolycarbosilane at 10 to 300 weight parts. It is therefore preferable toset the addition amount of polycarbosilane to 10 weight parts relativeto siloxane resin of 100 weight parts, in order to retain a highalkaline resistance.

[0063] Next, methods of producing low dielectric constant film materialsaccording to the fifth to eighth embodiments will be described. The lowdielectric constant film materials of the fifth to eighth embodimentsare produced by adding adamantane monophenol to the siloxane resin addedwith polycarbosilane of the first to fourth embodiment. The additionamount of polycarbosilane is 150 weight parts relative to siloxane of100 weight parts. The addition amount of adamantane monophenol is 0 to70 weight % relative to a mixture of siloxane resin and polycarbosilane.For the comparison sake, resin solution was produced which had 80 weight% of the addition amount of adamantane monophenol relative to themixture of siloxane resin and polycarbosilane.

[0064] Resin solution of the fifth to eighth embodiments and comparisonexamples was spin-coated to a silicon wafer surface to form lowdielectric constant films. Adamantane phenol is desorbed during a heattreatment for cross-linking to thereby obtain a porous film.

[0065]FIG. 3 is a graph showing a relation between relative dielectricconstants of porous low dielectric constant films and addition amountsof adamantane. The abscissa represents the addition amount of adamantanein the unit of “weight %”, and the ordinate represents the relativedielectric constant. In FIG. 3, white circle, white square, whitetriangle and white rhomboid symbols indicate the relative dielectricconstants of low dielectric constant films respectively made of the filmforming materials of the fifth to eighth embodiments. As apparent fromthe comparison between FIGS. 1 and 3, by making the film porous, therelative dielectric constant of the film can be made smaller.

[0066]FIG. 4 shows a relation between the adhesion degree of a porouslow dielectric constant film and the addition amount of adamantanemonophenol. The abscissa of FIG. 4 represents the addition amount ofadamantane monophenol in the unit of weight % relative to the mixture ofsiloxane resin and polycarbosilane, and the ordinate represents theadhesion degree in the unit of “N/cm²”. The adhesion degree was measuredby a method similar to that described with FIG. 2.

[0067] As the addition amount of adamantane monophenol is increased from70 weight % to 80 weight %, the adhesion degree of the film lowersrapidly. It is therefore preferable to set the addition amount ofadamantane monophenol to 70 weight % or less.

[0068] Next, a method of inspecting whether siloxane resin containspolycarbosilane will be described. Polycarbosilane has a main chain of(—Si—CH₂—Si—), whereas siloxane resin has a main chain of (—Si—O—Si—).

[0069] The bond (—Si—CH₂—Si—) can be confirmed from a peak of infraredspectra at 1080 to 1040 cm⁻¹. Although this peak partially overlaps apeak corresponding to the bond (—Si—O—Si—), these peaks are sharp sothat they can be distinguished from each other. Whether a low dielectricconstant film formed on a semiconductor substrate containspolycarbosilane can be judged from microscope infrared spectralanalysis.

[0070] Next, the structure of a semiconductor device and its manufacturemethod according to the embodiment will be described, the semiconductordevice using film forming materials of any of the first to eighthembodiments.

[0071]FIG. 5 is a cross sectional view of a semiconductor device havingaluminum (Al) wiring layers. A field oxide film 2 formed on the surfaceof a silicon substrate 1 defines an active region. A MOSFET 3 is formedin the active region. MOSFET 3 has a source region 3S, a drain region3D, a gate electrode 3G and a gate oxide film 31.

[0072] An inter-level insulating film 10 of SiO₂ and a stopper film 11of SiN are formed on the substrate, covering MOSFET 3. A contact hole 12is formed through the inter-level insulating film 10 in the areacorresponding to the drain region 3D. The side wall and bottom surfaceof the contact hole 12 are covered with a barrier layer 13 of TiN. Theinside of the contact hole 12 is fully buried with a plug 14 of tungsten(W).

[0073] The barrier layer 13 and plug 14 are formed by depositing a TiNlayer and a W layer over the whole substrate surface and by performingchemical mechanical polishing. Deposition of the TiN layer is performedby sputtering. Deposition of the W layer is performed by chemical vapordeposition (CVD) using tungsten hexafluoride and hydrogen.

[0074] First-layer wiring lines 20 are formed on the surface of thestopper film 11. The first-layer wiring line 20 has a three-layerstructure of a 50 nm thick TiN film 21, a 450 nm thick Cu-containing Alfilm 22 and a 50 nm thick TiN film stacked in this order. Pattering theTiN film and Al film is performed by plasma etching using hydrochloricacid gas. One first-layer wiring line 20 is electrically connected tothe W plug 14.

[0075] The surfaces of the first-layer wiring lines 20 and stopper film11 are covered with a liner film of SiO₂ having a thickness of 50 nm.The liner film 25 is formed by CVD using tetraethylorthosilicate (TESO)and oxygen.

[0076] A low dielectric constant film 26 is formed on the liner film 25.The low dielectric constant film 26 is formed by spin-coating siloxaneresin solution of one of the first to eighth embodiments. The lowdielectric constant film 26 is formed under the condition that it has athickness of 500 nm on the flat surface of the silicon substrate.

[0077] On the low constant dielectric constant film 26 a cap layer 27 ofSiO₂ having a thickness of 1000 nm is formed. The cap layer 27 is formedby CVD using TEOS and oxygen. The upper surface of the cap layer 27 isplanarized by CMP. CMP is performed until the total thickness of theliner film 25, low dielectric constant film 26 and cap layer 27 becomes1200 nm on the area where the first-layer wiring line 20 is notdisposed.

[0078] A via hole 28 is formed through the three layers including theliner film 25, low dielectric constant film 26 and cap layer 27. Thisvia hole 28 is formed by plasma etching using CF₄ and CHF₃. The sidewall and bottom surface of the via hole 28 are covered with a barrierlayer 29. The inside of the via hole 20 is fully buried with a W plug30. The barrier layer 29 and W plug 30 are formed by a method similar tothat used for the lower barrier layer 13 and plug 14.

[0079] On the cap layer 27, second-layer wiring lines 40 are formed. Asecond layer liner film 41, a low dielectric constant film 42 and a caplayer 43 are laminated covering the second-layer wiring lines 40. Theseliner film 41, low dielectric constant film 42 and cap layer 43 areformed by a method similar to that used for the first-layercorresponding films and layer.

[0080] In the multi-layer wiring structure shown in FIG. 5, the spacebetween adjacent wiring lines in the same layer is filled with the lowdielectric constant film. Since the low dielectric constant films 26 and42 are made of the material described in the above embodiments, amoisture absorption rate does not increase even if a process usingalkaline solution is performed, and a low dielectric constant ismaintained.

[0081]FIG. 6 is a cross sectional view of a semiconductor device havingcopper (Cu) wiring lines according to the ninth embodiment of theinvention. The structure from a silicon substrate 1 to a stopper film 11is the same as that of the corresponding components of the semiconductordevice shown in FIG. 5. In FIG. 6, constituent elements are representedby identical reference numerals to those corresponding elements shown inFIG. 5.

[0082] On the stopper film 11, a low dielectric constant film 50 isformed. The low dielectric constant film 50 is formed by using thematerial of the above-described embodiment under the condition that ithas a thickness of 450 nm on the flat surface of the silicon substrate.On this low dielectric constant film 50, a cap layer 51 of SiO₂ having athickness of 50 nm is formed. The cap layer 51 is formed by CVD usingTEOS and oxygen.

[0083] A first-layer wiring trench 52 is formed through the lowdielectric constant film 50 and cap layer 51. The first-layer wiringtrench 52 is formed by plasma etching using CF₄ and CHF₃. The uppersurface of the plug 14 is exposed on the bottom surface of thefirst-layer wiring trench 52.

[0084] The side wall and bottom surface of the first-layer wiring trench52 are covered with a barrier layer 53 of TaN having a thickness of 50nm. The inside of the wiring trench 52 is fully buried with thefirst-layer wiring line 54 of Cu. In the following, a method of formingthe barrier layer 53 and first-layer wiring line 54 will be described.

[0085] A TaN film is formed through sputtering on the whole surface ofthe substrate including the inner surface of the first-layer wiringtrench 52. A Cu film having a thickness of 50 nm is formed throughsputtering on the TaN film. By using this Cu film as an electrode, a Cufilm having a thickness of 600 nm is formed by electroplating.Unnecessary Cu and TaN films are removed by CMP to leave the barrierlayer 53 and first-layer wiring line 54 in the first-layer wiring trench52.

[0086] On the cap layer 51, a lamination structure is formed having a 50nm thick SiN diffusion preventing film 60, a low dielectric constantfilm 61, a 50 nm thick SiN stopper film 62, a low dielectric constantfilm 63, and a 50 nm SiN cap layer 64. The diffusion preventing film 60and stopper film 62 are formed by plasma CVD using silane and ammoniumgas. The low dielectric constant films 61 and 63 are formed under thecondition that they have thicknesses of 650 nm and 400 nm respectively,on the flat surface of the silicon substrate.

[0087] A via hole 68 is being formed through the diffusion preventingfilm 60 and low dielectric constant film 61. A second-layer wiringtrench 69 is being formed through the stopper film 62, low dielectricconstant film 63 and cap layer 64. The inner surfaces of the via hole 68and second-layer wiring trench 69 are covered with a barrier layer 70 ofTaN having a thickness of 50 nm. The second-layer wiring line 72 of Cufully buries the inside of the via hole 68 and second-layer wiringtrench 69. The second-layer wiring line 72 is formed by a dual-damascenemethod.

[0088] The dual-damascene method will be briefly described. First, thevia hole 68 is formed extending from the upper surface of the cap layer64 to the upper surface of the first-layer wiring line 54. Next, thesecond-layer wiring trench 69 is formed extending from the upper surfaceof the cap layer 64 to the upper surface of the low dielectric constantfilm 61. The barrier layer 70 and second-layer wiring line 72 are formedby a method similar to that used for the lower barrier layer 53 andfirst-layer wiring line 54.

[0089] The first-layer wiring line 54 and second-layer wiring line 72are surrounded by the low dielectric constant films 50, 61 and 63 sothat parasitic capacitance between wiring lines can be reduced. Sincethese low dielectric constant films 50, 61 and 63 are made of thematerial of the above-described embodiment, the moisture absorption ratedoes not increase and the dielectric constant can be maintained low,even if a process using alkaline solution is performed.

[0090] Next, with reference to FIG. 7, a semiconductor device and itsmanufacture method according to the tenth embodiment will be described.In the semiconductor device of the ninth embodiment shown in FIG. 6, thestopper film 62 of silicon nitride is disposed between the lowdielectric constant film 61 and its upper low dielectric constant film63. In the tenth embodiment, the stopper film 62 is not used but the lowdielectric constant film 63 directly contacts the low dielectricconstant film 61.

[0091] In the tenth embodiment, the low dielectric constant films 61 and63 are made of silica-containing porous material. Under the same etchingcondition, an etching rate of the upper low dielectric constant film 63is faster than that of the lower low dielectric constant film 61. Theother structures are similar to those of the ninth embodiment shown inFIG. 6.

[0092] The low dielectric constant films 61 and 63 contain siloxaneresin expressed, for example, by the following general chemical formula:

[0093] or ladder type siloxane resin expressed by the following generalchemical formula:

[0094] R₁₀ to R₁₂ represent hydrogen, oxygen or a monovalent hydrocarbongroup, and R₁₃ to R₁₆ represent hydrogen; fluorine or a monovalenthydrocarbon group. n₁ is an integer of 5 to 200, and X representshydrogen or silicon. n₂ is an integer of 5 to 100.

[0095] For the lower low dielectric constant film 61, at least one ofR₁₀ to R₁₂ or at least one of R₁₃ to R₁₆ is a phenyl group or ahydrocarbon group having 2 to 5 carbon atoms. For the upper dielectricconstant film 63, none of R₁₀ to R₁₂ are a hydrocarbon group having twoor more carbon atoms or none of R₁₃ to R₁₆ are a hydrocarbon grouphaving two or more carbon atoms.

[0096] The present inventors have found that an etching rate can bechanged by changing side chains of siloxane resin or ladder typesiloxane resin. Specifically, if the material has only hydrogen or amethyl group as side chains, the etching rate of the material by fluoricplasma becomes three times or more faster than such a material as atleast one side chain in one monomer unit is a phenyl group or ahydrocarbon group having two or more carbon atoms. In the tenthembodiment, the materials of the films 61 and 63 are selected so thatthe etching rate of the upper low dielectric constant film 63 becomesthree times or more faster than that of the upper low dielectricconstant film 61.

[0097] Next, an example of a method of producing the material of thelower low dielectric constant film 61 will be described.Tetraethoxysilane of 20.8 g (0.1 mol) and phenyltriethoxysilane of 20.4g (0.1 mol) are dissolved in methylisobutylketone of 37.2 g to obtainsolution of 200 ml. Nitric acid solution of 16.2 g (0.9 mol) at aconcentration of 400 ppm is dripped in ten minutes into the obtainedsolution and thereafter an aging process is performed for two hours.Tetraethoxysilane and phenyltriethoxysilane are therefore copolymerizedto produce siloxane resin. This siloxane resin has the composition thatat least one of R₁₀ to R₁₂ in the above-described general chemicalformula is a phenyl group and the others are oxygen atoms. Each oxygenatom is also bonded to a hydrogen atom or a silicon atom.

[0098] Next, magnesium nitrate of 5 g is added to the siloxane resinsolution to remove excessive water contents. Solvent containing ethanolis removed until the reaction solution reduces to 50 ml, by using arotary evaporator. Adamantane monophenol of 0.1 g is added to theproduced reaction solution. In this manner, siloxane resin solution forforming the lower low dielectric constant film 61 is produced.Adamantane monophenol is desorption agent for making the low dielectricconstant film porous.

[0099] Next, an example of a method of producing the material of theupper low dielectric constant film 63 will be described.Tetraethoxysilane of 20.8 g (0.1 mol) and methyltriethoxysilane of 17.8g (0.1 mol) are dissolved in methylisobutylketone of 39.6 g to obtainsolution of 200 ml. Nitric acid solution of 16.2 g (0.9 mol) at aconcentration of 400 ppm is dripped in ten minutes into the obtainedsolution and thereafter an aging process is performed for two hours.Tetraethoxysilane and methyltriethoxysilane are therefore copolymerizedto produce siloxane resin. This siloxane resin has the composition thatat least one of R₁₀ to R₁₂ in the above-described general chemicalformula is a methyl group and the others are oxygen atoms. Each oxygenatom is also bonded to a hydrogen atom or a silicon atom.

[0100] Next, similar to the synthesis of the lower low dielectricconstant film material, excessive water contents are removed and solventcontaining ethanol which is a byproduct of the aging process is removeduntil the reaction solution reduces to 50 ml. Adamantane monophenol of0.1 g is added to the produced reaction solution. In this manner,siloxane resin solution for forming the upper low dielectric constantfilm 63 is produced.

[0101] Next, a method of manufacturing the semiconductor device shown inFIG. 7 by using the above-described low dielectric constant filmmaterials will be described.

[0102] The processes up to forming the diffusion preventing film 60 aresimilar to the manufacture method of the semiconductor device of theninth embodiment shown in FIG. 6, and so the description thereof isomitted. On the diffusion preventing film 60, siloxane resin solution asthe lower low dielectric film material is spin-coated. Solvent is driedoff at a temperature of 200° C. An annealing process is performed for 30minutes at a temperature of 400° C. in a nitrogen atmosphere having anoxygen concentration of 100 ppm or lower. The lower low dielectricconstant film 61 made of silica-containing porous material and having athickness of 500 nm is therefore formed.

[0103] On the lower low dielectric constant film 61, siloxane resinsolution as the upper low dielectric film material is spin-coated.Solvent is dried off and an annealing process is performed to obtain theupper low dielectric constant film 63 having a thickness of 400 nm. Theconditions for solvent drying and annealing are the same as those usedfor forming the lower low dielectric constant film 61.

[0104] On the low dielectric constant film 63, the cap layer 64 ofsilicon oxide having a thickness of 50 nm is formed by chemical vapordeposition (CVD) using tetraethylorthosilicate (TEOS).

[0105] On the surface of the cap layer 64, a resist pattern having anopening corresponding to the via hole 68 is formed. A hole extendingfrom the upper surface of the cap layer 64 to the upper surface of thefirst-layer wiring line 54 is formed by using fluoric plasma using CF₄and CHF₃ as source materials. After the resist pattern is removed, aresist pattern having an opening corresponding to the second-layerwiring trench 69 is newly formed on the surface of the cap layer 64.

[0106] By using fluoric plasma using C₂F₆ and O₂ as source materials,the upper low dielectric constant film 63 is etched to form thesecond-layer wiring trench 69. This etching is performed under theconditions of a C₂F₆ flow rate of 40 sccm, an O₂ flow rate of 10 sccm,an input power of 200 W for generating inductive coupling plasma, and agas pressure of 5.32 Pa (40 mTorr). Under these etching conditions, theetching rate of the upper low dielectric constant film 63 is about 100nm/min, whereas the etching rate of the lower low dielectric constantfilm 61 is about 30 nm/min. Since the etching rate of the lower lowdielectric constant film 61 is slower than the etching rate of the upperlow dielectric constant film 63, the etching can be stopped with goodcontrollability when the upper surface of the lower low dielectricconstant film 61 is exposed, even if an etching stopper layer is notprovided. In order to stop the etching with good controllability, it ispreferable that under the same etching conditions, the etching rate ofthe upper low dielectric constant film 63 is two times or more fasterthan the etching rate of the lower low dielectric constant film 61.

[0107] After the resist pattern is removed, the barrier layer 70 andsecond-layer wiring line 72 are formed. The barrier layer 70 andsecond-layer wiring line 72 are formed by a method similar to the firstembodiment shown in FIG. 6.

[0108] In the tenth embodiment, a silicon nitride film having arelatively high dielectric constant is not disposed between the lowerand upper low dielectric constant films 61 and 63. Therefore, aparasitic capacitance between wiring lines can be reduced further. Anelectrostatic capacitance between two second-layer wiring lines 72disposed in parallel was measured to calculate an effective relativedielectric constant which was about 2.5. In contrast, an effectiverelative dielectric constant of the ninth embodiment shown in FIG. 6 was2.8.

[0109] An evaluation sample was manufactured to measure a relativedielectric constant of a lamination structure including low dielectricconstant films. This sample is a lamination of a lower low dielectricconstant film of 300 nm in thickness, an upper low dielectric constantfilm of 300 nm in thickness and a silicon oxide film of 50 nm inthickness made of TEOS, respectively stacked upon a silicon substrate.An Au film having a diameter of 1 mm and a thickness of 100 nm wasformed on the surface of the uppermost silicon oxide film, and anelectrostatic capacitance between the silicon substrate and Au film wasmeasured. From this measurement result, a relative dielectric constantof the three-layer structure including the two low dielectric constantfilms and one silicon oxide film was calculated. The relative dielectricconstant was 2.4. In contrast, a relative dielectric constant of afour-layer structure including two low dielectric constant films, onesilicon nitride film having a thickness of 50 nm disposed between thelow dielectric constant films and one silicon oxide film was 2.7. Asilicon oxide film made of TEOS has a relative dielectric constant ofabout 4, and the silicon nitride film has a relative dielectric constantof about 7.

[0110] As seen from the tenth embodiment and the above-describedevaluation results, if the etching stopper film made of silicon nitrideis not disposed, a relative dielectric constant of a laminationstructure including low dielectric constant films can be reduced.

[0111] The material of the upper low dielectric film may be: siloxaneresin used by the tenth embodiment; resin produced by a sol-gel methodusing tetraalkoxysilane, trialkoxysilane, methyltrialkoxysilane or thelike as source material; resin produced by a sol-gel method using amixture of these source materials; resin produced by a sol-gel methodusing tetraalkoxysilane and dimethylalkoxysilane as source materials; orother resin. Ladder type resin may be hydrogen silsesquioxane,methylsilsesquioxane, fluorine-containing hydrogen silsesquioxane or thelike.

[0112] The material of the lower low dielectric film may be: siloxaneresin used by the tenth embodiment; and resin produced by a sol-gelmethod using phenyltrialkoxysilane. Ladder type resin may bephenylsilsesquioxane or the like. Resin containing a hydrocarbon groupwith 2 to 5 carbon atoms as at least one of side chains may be resinproduced by a sol-gel method using at least one source material selectedfrom a group consisting of ethyltrialkoxysilane, propyltrialkoxysilane,normal-butyltrialkoxysilane, and tertiary-butyltrialkoxysilane.

[0113] The present invention has been described in connection with thepreferred embodiments. The invention is not limited only to the aboveembodiments. It is apparent that various modifications, improvements,combinations, and the like can be made by those skilled in the art.

What we claim are:
 1. A low dielectric constant film forming materialcomprising siloxane resin and polycarbosilane dissolved together.
 2. Alow dielectric constant film forming material according to claim 1 ,wherein an average molecular weight of the siloxane resin is 1,000 to500,000, and the polycarbosilane is dissolved by 10 to 300 weight partsrelative to siloxane resin of 100 weight parts.
 3. A low dielectricconstant film forming material according to claim 1 , wherein organiccompound is further dissolved which is desorbed by heat or light, and anamount of the organic compound is 10 to 70 weight % relative to amixture of the siloxane resin and the polycarbosilane.
 4. A lowdielectric constant film forming material comprising 100 weight parts ofsiloxane resin and 10 to 300 weight parts of polycarbosilane dissolvedin solvent, wherein the siloxane resin is expressed by a generalchemical formula of:

(R₁ to R₃ represent hydrogen, oxygen or a monovalent hydrocarbon group,X represents hydrogen or silicon, and n₁ is an integer of 5 to 200), or;

(R₄ to R₇ represent hydrogen, fluorine or a monovalent hydrocarbongroup, n₂ is an integer of 5 to 100, wherein at least one of R₄ to R₇ ishydrogen), and the polycarbosilane is expressed by a general chemicalformula of:

(R₈ and R₉ represent hydrogen or a monovalent hydrocarbon group, and mis an integer of 20 to 1000).
 5. A low dielectric constant film materialaccording to claim 4 , wherein organic compound is further dissolvedwhich is desorbed by heat or light, and an amount of the organiccompound is 10 to 70 weight % relative to a mixture of the siloxaneresin and the polycarbosilane.
 6. A low dielectric constant filmcontaining siloxane resin and polycarbosilane bonded to the siloxaneresin.
 7. A semiconductor device comprising: a semiconductor substrate;and a low dielectric constant film made of low dielectric constantmaterial containing siloxane resin and polycarbosilane bonded to thesiloxane resin.
 8. A semiconductor device comprising: a semiconductorsubstrate; a first film formed on a surface of said semiconductorsubstrate and made of a first silica-containing porous material; and asecond film directly formed on said first film and made of a secondsilica-containing porous material, the second silica-containing porousmaterial having an etching rate different from an etching rate of thefirst silica-containing porous material under a same etching condition.9. A semiconductor device according to claim 8 , wherein a fasteretching rate of one of the first silica-containing porous material istwo times or more faster than a slower etching rate of the other.
 10. Asemiconductor device according to claim 8 or 9 , wherein the first andsecond silica-containing porous materials contain siloxane resinexpressed by a general chemical formula of:

(R₁₀ to R₁₂ represent hydrogen, oxygen or a monovalent hydrocarbongroup, n₁ represents an integer of 5 to 200, and X represents hydrogenor silicon), or:

(R₁₃ to R₁₆ represent hydrogen, fluorine or a monovalent hydrocarbongroup, n₂ is an integer of 5 to 100, wherein at least one of R₁₃ to R₁₆is hydrogen).
 11. A semiconductor device according to claim 10 , whereinfor the first silica-containing porous material, at least one of R₁₀ toR₁₂ is a phenyl group or a hydrocarbon group having two to five carbonatoms or at least one of R₁₃ to R₁₆ is a phenyl group or a hydrocarbongroup having two to five carbon atoms, and for the secondsilica-containing porous material, none of R₁₀ to R₁₂ are a hydrocarbongroup having two or more carbon atoms or none of R₁₃ to R₁₆ are ahydrocarbon group having two or more carbon atoms.
 12. A semiconductordevice according to claim 8 , wherein an etching rate of the secondsilica-containing porous material is faster than an etching rate of thefirst silica-containing porous material, and the semiconductor devicefurther comprises: a trench formed in said second film and having adepth greater than a thickness of said second film; a via hole formedthrough said first film, said via hole being partially overlapped bysaid trench; and a conductive wiring line burying an inside of said viahole and trench.
 13. A semiconductor device according to claim 10 ,wherein an etching rate of the second silica-containing porous materialis faster than an etching rate of the first silica-containing porousmaterial, and the semiconductor device further comprises: a trenchformed in said second film and having a depth greater than a thicknessof said second film; a via hole formed through said first film, said viahole being partially overlapped by said trench; and a conductive wiringline burying an inside of said via hole and trench.
 14. A method ofmanufacturing a semiconductor device, comprising the steps of: forming afirst film made of a first silica-containing porous material on asurface of a semiconductor substrate; forming a second film of a secondsilica-containing porous material directly on a surface of the firstfilm, an etching rate of the second silica-containing porous materialbeing faster than an etching rate of the first silica-containing porousmaterial; forming a trench having a depth greater than a thickness ofthe second film and a via hole through the first film, the via holebeing partially overlapped by the trench; and burying a conductivematerial in the via hole and trench.
 15. A method of manufacturing asemiconductor device according to claim 14 , wherein said step offorming the trench and via hole includes the steps of: forming a holethrough the first and second films; and forming the trench by etching aregion being partially overlapped by the hole, from an upper surface ofthe second film to at least an upper surface of the first film.
 16. Asemiconductor device comprising: a semiconductor substrate; a first filmformed on a surface of said semiconductor substrate and made of a firstsilica-containing porous material; and a second film directly formed onsaid first film and made of a second silica-containing porous material,wherein the first and second silica-containing porous materials containsiloxane resin expressed by a general chemical formula of:

(R₁₀ to R₁₂ represent hydrogen, oxygen or a monovalent hydrocarbongroup, n₁ represents an integer of 5 to 200, and X represents hydrogenor silicon), or:

R₁₃ R₁₆ represent hydrogen, fluorine or a monovalent hydrocarbon group,n₂ is an integer of 5 to 100, wherein at least one of R₁₃ to R₁₆ ishydrogen), for the first silica-containing porous material, at least oneof R₁₀ to R₁₂ is a phenyl group or a hydrocarbon group having two tofive carbon atoms or at least one of R₁₃ to R₁₆ is a phenyl group or ahydrocarbon group having two to five carbon atoms, and for the secondsilica-containing porous material, none of R₁₀ to R₁₂ are a hydrocarbongroup having two or more carbon atoms or none of R₁₃ to R₁₆ are ahydrocarbon group having two or more carbon atoms.
 17. A method ofmanufacturing a semiconductor device comprising the steps of: forming afirst film made of a first silica-containing porous material on asurface of a semiconductor substrate; forming a second film made of asecond silica-containing porous material directly on a surface of thefirst film; forming a trench having a depth greater than a thickness ofthe second film and a via hole through the first film, the via holebeing partially overlapped by the trench; and burying a conductivematerial in the via hole and trench, wherein the first and secondsilica-containing porous materials contain siloxane resin expressed by ageneral chemical formula of:

(R₁₀ to R₁₂ represent hydrogen, oxygen or a monovalent hydrocarbongroup, n₁ represents an integer of 5 to 200, and X represents hydrogenor silicon), or:

(R₁₃ to R₁₆ represent hydrogen, fluorine or a monovalent hydrocarbongroup, n₂ is an integer of 5 to 100, wherein at least one of R₁₃ to R₁₆is hydrogen), for the first silica-containing porous material, at leastone of R₁₀ to R₁₂ is a phenyl group or a hydrocarbon group having two tofive carbon atoms or at least one of R₁₃ to R₁₆ is a phenyl group or ahydrocarbon group having two to five carbon atoms, and for the secondsilica-containing porous material, none of R₁₀ to R₁₂ are a hydrocarbongroup having two or more carbon atoms or none of R₁₃ to R₁₆ are ahydrocarbon group having two or more carbon atoms.